Why do incubator's need humidity?

Bear with me here, I have had chickens for a few years but no NOTHING about hatching an egg. So the question is: Why is humidity needed in an incubator, but not when being sat on by momma? Please be kind, I only ask to know more about all this chicken stuff.

This is a great place to ask questions--everyone has been very nice--so ask away!

Momma hen is a source of humidity. And she can hold moisture with her insulating feathers. I don't think that birds sweat per se, but all skin looses moisture, she just doesn't have the super capacity to loose moisture thru sweat glands.

She is amazing that she can keep a constant temperature and constant moisture level.

I see! My husband and i were talking last night. We are building a new coop and possibly a coop just for banties and silkies...and I thought hmmm....maybe it's time to do more. I will definitely read and read and read before even buying what's needed, but the question has been on my mind all day!